The invention relates to diagnosing or determining parameters for installations for detecting open defects in the surfaces of parts, in particular metal parts, by using the sweating technique.
Sweating is a well-known technique for non-destructive testing of parts that might present open surface defects, such as cracks or fissures, for example. A penetrating composition containing a detector or indicator substance is applied to the surface of the part to be inspected. The detector or indicator substance typically comprises a colored or fluorescent compound that shines when exposed to appropriate light. After the penetrating composition has been applied, the surface of the part is washed so as to be cleaned, and then it is dried. A composition containing an absorbent substance or developer is then deposited on the surface of the part. By capillarity, the developer causes any indicator substance that has penetrated into open defects to return to the surface (where it is “sweated out”). Illuminating the surface by light appropriate for the indicator substance, typically ultraviolet (UV) light, serves to show up any defects, thus enabling the part to be inspected by visual examination.
A description of known methods of detecting surface fissures using penetrating substances can be found in the article by H. J. Stumm entitled “Der Nachweis von Oberflachenrissen mit Hilfe von Eindringmitteln”, G-I-T Fachzeitschrift für das Laboratorium, Darmstadt, DE, Vol. 17, No. 9, 1973, pp. 955, 957-958.
The quality of detection is sensitive to numerous installation set-up parameters and it is necessary to undertake regular verification that the installation is operating properly. For this purpose, use is made of standard testpieces that have a surface presenting patterns that are representative of the types of open defect that are to be detected. The standard testpiece, optionally together with a batch of parts for inspection, is caused to pass through the installation and the patterns developed at the end of the sweating process on the surface of the standard testpiece are verified for compliance with the patterns whose geometrical characteristics are already known.
Standard testpieces can be used not only for verifying proper operation of an installation for detecting defects by sweating, but also for determining the parameters of such an installation, i.e. for optimizing the settings of the various parameters that have an influence on the quality of detection, as proposed in patent document EP 0 650 045.